we have concentrated during this fiscal year on the demonstration of rapid structural changes in nervous tissues associated with synaptic activities. the electric organ of the electric ray, Torpedo is known to be well suited for t he study of the mechanism of synaptic transmission because of its homogeneity and its massive cholinergic innervation. Using sensitive mechanical and optical devices fabricated in this laboratory we have found that electric stimulation of one of the nerves entering into the organ produces rapid swelling followed by slow shrinkage of the organ. Furthermore by use of special heat-sensors which we have designed an fabricated we were able demonstrate that there is rapid heat production followed by slow heat absorption by the organ. Note that the hear-sensor employed by previous investigators had a response-time of about 100 msec it was necessary for us to construct a new type of heat-sensor which have a high time-resolution and are completely shielded from the electric organ to direct stimulation. Using a small aluminum lever connected to a small strip of the organ slice we have established that the electrocytes in the organ respond to the chemical transmitter(acetylcholine released from the nerve endings) with rapid swelling followed by slow shrinkage. In addition we have found that generation of a postsynaptic potential in the slice is accompanied by a rapid heat production followed by a slow heat absorption. We believe that these findings give strong support to the theory of chemical excitation put forward by Neumann,, Nachmansohn and Katchalsky (1973- 1976); this theory proposes that the receptor protein molecules have a large binding capacity for Ca-ions and the chemical transmitter causes release of the bound Ca-ions and the chemical transmitter causes release of the bound Ca-ions from the receptors molecules. A more detailed examination of the validity of this theory is in progress.